This was a question asked on this similar subject "Is there legislation dealing with the problem of high hedges and does the Borough Council have any powers concerning high hedges? The law giving local councils powers to deal with complaints about high hedges is contained in Part 8 of the Anti-social Behaviour Act 2003 ("the Act") and the High Hedges (Appeals) (England) Regulations 2005 ("the Appeal Regulations")"
Your local council will tell you of local rules but I think usually its now 6 feet if its a continuos hedge? Hope this helps not an easy area?

Planting Leylandii along a neighbours border can be a cause of future nasty disputes as they grow so fast. Even murder. Read Wikipaedia. As pointed out above, there are no actual measurements involved. Best advice is ,if you intend to plant on a neighbours boundary......DON'T do it. As a specimen tree in 'grounds' away from neighbours.....fantastic.

If you do plant them it will be costly to control them and also suck all the goodness from your garden. Or have you already got them?